The rubber composition of the tread determines the road properties of a tire, in particular a pneumatic vehicle tire, to a large extent. The rubber mixtures used mainly in the heavily mechanically stressed areas of belts, hoses, and straps are also largely responsible for the stability and durability of these rubber articles. For this reason, the standards for these rubber mixtures for pneumatic vehicle tires, straps, belts, and hoses are very high.
By means of partial or complete replacement of the filler carbon black by silica in rubber mixtures, the road properties of tires, for example, have been improved overall to a higher level in recent years. However, the known conflicting objectives of tire properties with respect to one another continue to be present in silica-containing tread mixtures as well. Thus, an improvement in wet grip and dry braking generally continues to cause deterioration of rolling resistance, winter properties, and wear behavior. These properties are important criteria for quality in technical rubber articles such as straps, belts, and hoses as well.
In vehicle tires in particular, a wide variety of attempts have been made to positively influence the properties of tires by varying polymer components, fillers, and other aggregates, particularly in the tread mixture. The focus here is primarily on the properties of rolling resistance and wear. It must here be borne in mind that an improvement in one tire property often causes worsening of another property.
In a given mixing system, for example, there are various known possibilities for optimizing rolling resistance. These include reducing the degree of filling, changing the polymer system, and reducing the glass transition temperature Tg of the rubber mixture. All of the aforementioned measures result in a decline in the wear properties and/or wet grip properties and/or tear properties of the mixture in question.
In the present document, the term vehicle tires is understood to refer to pneumatic vehicle tires, solid rubber tires, and two-wheel vehicle tires.
In particular, affecting the glass transition temperature of the rubber mixture used by selecting suitable polymer systems is frequently discussed in expert circles.
In this connection, it is known that the glass transition temperature of otherwise identical mixture components of two rubber mixtures is determined by the glass transition temperature of the polymer(s) used. The higher the glass transition temperature of a polymer, the higher the glass transition temperature of the rubber mixture as well, and the less favorable the rolling resistance behavior of the rubber mixture. Good indicators for the rolling resistance behavior of rubber mixtures are rebound elasticity at 60 to 70° C. and hysteresis loss values, expressed by tan δ at 60 to 70° C.
It is generally known that 1,4-polybutadiene rubber has an extremely low glass transition temperature of approximately −105° C., which makes this rubber suitable for improving the rolling resistance behavior of rubber mixtures. However, it is also known that this considerably impairs the wet grip behavior of the rubber mixture.
Another known method of influencing tire properties such as wear, wet grip performance, and rolling resistance is the use of different styrene-butadiene copolymers with differing styrene and vinyl contents and differing modifications in the rubber mixtures, wherein the above-described problem of conflicting objectives arises in this case as well.
WO 2009007167 A1 discloses the use of two different polymers with differing glass transition temperatures in order to improve wet grip.
Also for the purpose of improving wet grip, EP 0659821 A1 discloses the use of 20 to 80 phr of diene rubber, in this specific case natural rubber, and 80 to 20 phr of styrene-butadiene copolymer having a glass transition temperature between −50° C. and −25° C. The use of 10 to 50 phr of diene rubber, here styrene-butadiene rubber, having a glass transition temperature of less than −45° C. to improve the ratio of dry to wet gripping is described in EP 1253170 A1. In U.S. Pat. No. 6,812,288, on the other hand, 5 to 40 phr of styrene-butadiene copolymer having a glass transition temperature of −35° C. or higher and 95 to 60 phr of diolefin rubber having a glass transition temperature of −20° C. or less are used to improve the shock-absorption properties (“vibration-isolating properties”) of the rubber mixture.
DE 40 01 822 C2 describes a rubber mass comprising 10 to 100 parts by weight of a solution-polymerized styrene-butadiene rubber having a vinyl content of 20 to 70 wt % and a styrene content of 54.5 to 65 wt %, 0 to 90 parts by weight of an emulsion-polymerized styrene-butadiene rubber having a glass transition temperature of at least −60° C. and a styrene content of 20 to 65 wt %, and at least 70 parts by weight of carbon black, which are mixed into this rubber mass. This rubber mass is intended for use in running surfaces of high-performance tires with major hysteresis loss, high heat resistance, and a substantial grip.
Moreover, U.S. Pat. No. 5,901,766 describes a pneumatic tire with a sulfur-vulcanizable composition that is characterized by containing 50 to 90 phr of a rubber having a glass transition temperature in the range of −80° C. to −110° C., 10 to 50 phr of at least one rubber having a glass transition temperature in the range of −79° C. to +20° C., and 15 to 50 phr of a resin that is not a rubber. This mixture shows improved laboratory properties, which correlate with improved tire wear and concomitant improvement in grip and road behavior.
However, the improvement in grip behavior due to increased hysteresis loss, that is, greater than tan δ at 0° C., is known to be accompanied by deterioration of rolling resistance properties, that is, shock absorption during driving, which can be seen, for example, in U.S. Pat. No. 5,901,766 from the simultaneous increase in tan δ at 60° C. in ESBR and BR-containing rubber mixtures.
In order to optimize rolling resistance behavior or optimize various other properties of rubber mixtures that are relevant for use in tires without impairing rolling resistance behavior, the method is known of functionalizing the diene rubber used in such a way that binding to the filler(s) takes place.
Thus, for example, U.S. Pat. No. 8,450,424 discloses a rubber mixture that contains at least one aliphatic and/or aromatic hydrocarbon resin, at least one filler, and at least one functionalized diene rubber, whose functionalization takes place along and/or at the end of the polymer chain and allows binding to fillers. The hydroxy groups in Table 1 are disclosed as functionalizations for binding of the polymers to silica.
U.S. Pat. No. 8,426,512 discloses a rubber mixture that contains equal amounts of silica, carbon black, and functionalized polymers, with the use of 50 phr of polybutadiene functionalized with siloxy or siloxy aldimine groups instead of 50 phr of unfunctionalized polybutadiene being disclosed, among other uses. Such a rubber mixture shows improved rolling resistance indicators (rebound 100° C.), while the wet grip properties become poorer (rebound 23° C.). The effect on tear properties, in particular tear propagation properties, is not disclosed in U.S. Pat. No. 8,426,512.
EP 1963110 B1 discloses polymers modified with silane sulfide having a glass transition temperature of −23 to −28° C., which make it possible to achieve in a rubber mixture low values for the loss factor tan delta (tan δ) at 60° C., with the other properties otherwise being well-balanced.
U.S. Pat. No. 7,241,842 discloses a styrene-butadiene rubber that bears polyorganosiloxane groups containing epoxy groups as a functionalization, with three or more polymer chains being linked to one polyorganosiloxane group. When this polymer is combined with an unfunctionalized butadiene rubber in a silica-containing rubber mixture, this provides improved rolling resistance, wear and wet grip properties.